The invention provides water-blown polyurethane sealing devices and compositions for producing such devices. More particularly, the invention provides isocyanate prepolymer compositions and polyurethane compositions containing such prepolymer compositions, both compositions being especially suitable for the preparation of polyurethane sealing devices having advantageous performance properties such as improved hardness and compression characteristics.
Various aspects of the polyurethane industry have seen increasing government regulation in the area of acceptable blowing agents. This has been especially true for those involved in the production of polyurethane foams and molded microcellular polyurethane articles. In particular, due to environmental concerns relating to the earth's atmosphere and ozone levels, it is now highly desirable to replace certain physical blowing agents with alternative blowing agents.
Although research continues in the development of new blowing agents, renewed attention has been directed toward the use of water as a principal blowing agent. Desirable because of its low cost, ready availability, and minimal safety hazards, the use of water as a blowing agent nonetheless presents several problems.
Polyurethane polymers of the sort to which the instant invention is directed may generally be identified as having primary chains characterized by a segmented or block co-polymer structure. The segments are generally identified as being either "hard" or "soft" segments. Soft segments are the residual of long chain polyether or polyester polyols and are generally more flexible. Hard segments are formed from the residual of the diisocyanate and a short chain diol or chain extender.
The use of water as the principal blowing agent in the production of molded, microcellular polyurethane foams results in the incorporation of short brittle segments in the polymer matrix. These brittle segments are less forgiving to stress and detract from the overall physical properties of the resulting polymer. Properties such as flexibility, abrasion resistance, hardness, strength, and fatigue resistance appear to be particularly vulnerable. In addition, principally water blown polyurethane compositions appear to be somewhat inferior with respect to their ability to perform in a variety of processing latitudes, i.e. exhibit adequate flowability and/or demold times in molding operations.
As a result of these problems, certain segments of the polyurethane industry have found it difficult to adapt their formulations to 100% or principally water blown. The switch to water blown polyurethane formulations has been most difficult where the resulting microcellular polyurethane article must meet vigorous performance requirements.
An illustrative example is the production of sealing devices. The term "sealing device" is intended to encompass devices such as gaskets, air filter seals and end caps, as well as covered gaskets for the same. Common applications for such sealing devices include use on all types of motor driven vehicles, including, but not limited to, gas and diesel powered cars, trucks, and heavy equipment.
The operating environments of these devices often require that they function in extremes of temperature and pressure. Depending upon the end-use application, such sealing devices are often subjected to corrosive and/or structurally altering materials such as oil and gasoline. Performance properties evidencing strength, hardness, and compression characteristics have been found to be of paramount importance in the design and production of sealing devices. In particular, the characteristics measured by compression force deflection, tensile strength, and compression set have been found to be particularly vital. In general, measures taken to improve one of these properties often have detrimental effects on one or both of the other properties. An optimum balance between the three properties has been difficult to achieve in sealing devices.
As a result, it has been particularly difficult to provide water blown polyurethane compositions capable of providing molded microcellular polyurethane foams having the performance and processing properties required of commercially acceptable sealing devices.
The prior art has unsuccessfully attempted to address these problems. Several attempts have focused on the use of particular isocyanate prepolymers.
For example, U.S. Pat. No. 4,287,307 to Hostettler, discloses isocyanate-terminated prepolymers which are obtained by the reaction of (i) symmetrical aliphatic diisocyanates, cycloaliphatic diisocyanates, aromatic diisocyanates, or mixtures of such with their carbodiimide derivatives, with (ii) hydroxyl-terminated mono- or polyesters, or certain glycols or polyoxyalkylene glycols. However, the properties of the resulting foams are such that one or more plasticizers having one of four delineated structures must be used to obtain foams which are useful as shoe soles, vibration dampening devices, floatation devices, gaskets and the like. Also, example 4 of the '307 patent teaches that undesirable performance characteristics are obtained with water blown foams.
U.S. Pat. No. 4,321,333 to Alberino et al, discloses the use of a organic polyisocyanate (A) in the production of polyurethane polymers. Polyisocyanate (A) is disclosed as a blend of a prepolymer (a) and a liquified methylene bis(phenylisocyanate) (b). The prepolymer (a) is a reaction product of methylene bis(phenylisocyanate) with a polyol mixture comprising (1) a polyol having a molecular weight of at least 1,000, and (2) a polyol having a molecular weight of about 115 to 300. The '333 patent teaches that the additive prepolymer (a) in the isocyanate blend acts as an annealer to allow the curing of molded polyurethanes at temperatures lower than those prior art molded polyurethanes while resulting in improved green strength. However, the resultant foams do not appear to have the physical properties required of commercially acceptable sealing devices. In particular, the '333 patent fails to disclose important performance properties such as compressive force deflection and compression set.
In U.S. Pat. No. 4,374,210 to Ewen et al, the preparation of polyurethane-polyureas using aromatic diamine extenders is disclosed wherein certain prepolymers of 4,4'-methylene bis(phenylisocyanate) are used in place of 4,4'-methylene bis(phenylisocyanate) itself. The particular prepolymers are derived from the reaction of 4,4'-methylene bis(phenylisocyanate) with polyoxyethylene polyoxypropylene polyether diols and triols having molecular weights from 1,000 to 10,000, polytetramethylene glycols having molecular weights from 600 to 5,000, and polyester diols and triols having molecular weights from 500 to 8,000. However, this invention is directed toward the production of polyurea-urethanes requiring the use of aromatic diamines. The resultant moldings do not have the performance properties required of sealing devices.
U.S. Pat. No. 4,559,366 to Hostettler, relates to improved integral skin microcellular polyurethane elastomers intended for use as shoe soles. The disclosed improvement relates to the use of quasi-prepolymers which are normally liquid, may have crystalline points as low as -20.degree. C., and are derived from the reaction of (1) a mixture of normally solid diphenylmethane diisocyanate, carbodiimide and/or uretoneimine and (2) poly(oxyethyleneoxypropylene) polyols. However, the disclosures of the '366 patent are particularly directed toward the production of integral skin microcellular polyether polyurethane elastomer products having specific gravities in the range of about 0.4 to 0.7 gm/cm.sup.3. In addition, the patent expressly teaches that the elimination of water as a secondary blowing agent is preferred, and that blowing is most preferably effected with halocarbons.
Finally, published PCT application WO 92/22595 by the Dow Chemical Company, discloses a process for preparing polyurethane elastomer from a soft-segment isocyanate-terminated prepolymer. The prepolymer is obtained by reacting a molar excess of polyisocyanate having at least 70 weight percent 4,4'-methylene diphenylisocyanate, with an isocyanate-reactive composition containing (a) a branched diol or triol having a molecular weight of from 60 to 300, and (b) a polyoxyalkylene polyol or mixtures thereof having an average functionality of from 2 to 4 and a molecular weight of from 3000 to 12000, wherein the parts by weight ratio of (a):(b) is from 0.01:1 to 0.25:1. However, the requirement of the particular branched diols or triols adds to the cost of the composition. The resultant foams fail to provide the performance properties required of sealing devices due to the brittleness incorporated by the short claim diols in the polymer backbone.
Thus, it would be highly desirable to provide a principally water blown polyurethane composition capable of providing molded microcellular polyurethane articles having advantageous performance properties such as improved hardness, strength, and compression which are especially suited for use as sealing devices.
Accordingly, it is an object of this invention to provide principally water blown polyurethane compositions which are particularly suitable for the preparation of molded microcellular polyurethane articles having advantageous performance properties such as improved hardness, strengths, and compression characteristics.
More particularly, it is an object of this invention to provide principally water blown polyurethane compositions which are particularly suited for the preparation of molded microcellular polyurethane sealing devices exhibiting an optimum balance between the values for compression force deflection, tensile strength, and compression set.
It is a further object of this invention to provide isocyanate prepolymer compositions particularly suitable for use in polyurethane compositions which are solely or principally water blown and which are used in the production of molded microcellular polyurethane articles.
It is an additional object of the invention to provide isocyanate prepolymer compositions intended for use in principally water blown polyurethane compositions used in the production of microcellular molded polyurethane articles exhibiting desirable values for compression force deflection, tensile strength, and compression set.
Finally, it is an object of this invention to provide molded polyurethane articles and a process for making such, wherein the resulting molded polyurethane articles have an optimum balance between hardness, strength, and compression characteristics, such as desirable compression set, tensile strength and compression force deflection values.